It has been known from old days that silver and copper inhibit pathogenic bacteria, such as Escherichia coli and salmonellae, from proliferating.
Recently, materials to which the bacterial proliferation-inhibiting activity (hereinafter referred to as "antibacterial properties") is imparted using such metals have been developed. For example, Japanese Unexamined Patent Application Publication No. 8-49085 discloses a stainless steel sheet having superior antibacterial properties in which a metal layer or an alloy layer of Cr, Ti, Ni, Fe, etc., containing Ag and/or Cu is formed on the surface of a stainless steel base by magnet sputtering. In the steel sheet, it is preferable that the alloy layer or the metal layer containing 19 to 60% by weight of Ag be formed.
Japanese Unexamined Patent Application Publication No. 8-156175 also discloses a coated steel sheet which can inhibit bacteria from proliferating, in which a pigment containing silver is applied.
However, with respect to the method for forming the alloy layer or the metal layer containing the antibacterial metal on the surface of the steel sheet or to the method for applying the pigment containing the antibacterial metal to the surface of the steel sheet described above, the layer containing the antibacterial metal may be separated or removed by drawing or grinding of the surface, thus canceling the effects thereof, which is disadvantageous. Additionally, when used for kitchen utensils, etc., since surfaces are scrubbed by cleaning, it is not possible to maintain antibacterial properties for a long period of time, which is also disadvantageous.
Japanese Unexamined Patent Application Publication No. 8-239726 discloses a material with antibacterial properties and marine life resistance, which contains 10 to 80% by weight of iron, 1 to 10% by weight of aluminum, and preferably, further contains 1 to 15% by weight of at least one metal selected from the group consisting of chromium, nickel, manganese, and silver, the balance being copper and incidental impurities. However, since the above material is a copper-based alloy or iron-based alloy containing 1 to 10% of aluminum, the workability is decreased, and in particular, it gives rise to a problem when used for tableware, kitchen utensils, electric components, etc., in which the material is formed into sheets.
In view of the problems described above, Japanese Unexamined Patent Application Publication No. 8-104953 discloses an austenitic stainless steel with increased antibacterial properties, containing 1.1 to 3.5% by weight of Cu, Japanese Unexamined Patent Application Publication No. 10-259457 discloses an austenitic stainless steel with increased antibacterial properties, containing 0.5 to 4.0% by weight of Cu and 0.05 to 1.0% by weight of Ag, Japanese Unexamined Patent Application Publication No. 8-104952 discloses a martensitic stainless steel with increased antibacterial properties, containing 0.3 to 5.0% by weight of Cu, and Japanese Unexamined Patent Application Publication No. 9-170053 discloses a ferritic stainless steel with increased antibacterial properties, containing 0.4 to 3.0% by weight of Cu.
However, in order to display antibacterial properties, Cu, as ions, must be dissolved out of the surface of the steel sheet. The dissolution of Cu, as ions, means that a passivation film is destroyed at the spot, and even though antibacterial properties are improved, corrosion resistance is significantly deteriorated. Therefore, in the stainless steel to which a large amount of Cu is added, it is difficult to simultaneously acquire antibacterial properties and corrosion resistance.
Japanese Unexamined Patent Application Publication No. 10-259456 discloses an antibacterial stainless steel sheet containing at least 8% by weight of Cr and 0.05 to 1.0% by weight of Ag, in which the Ag phase having a minor axis of 10 .mu.m or less is dispersed in the matrix at an areal rate of 0.03% or more.
However, in the technique described in Japanese Unexamined Patent Application Publication No. 10-259456, it is substantially difficult to restrict the minor axis of the Ag phase to 10 .mu.m or less, and in order to set the areal rate of the Ag phase having the minor axis of 10 .mu.m or less at 0.03% or more, a large amount of Ag must be added, resulting in a decrease in workability and corrosion resistance, and also a degradation in surface properties.
Although a stainless steel product is generally worked into an end product after being subjected to the descaling process by acid pickling, in the case of a BA-finished stainless steel product, after cold rolling is performed, annealing (bright annealing treatment, i.e., BA treatment) is performed in a reducing atmosphere, and the BA-finished stainless steel product is handled as an end product without being subjected to the descaling process by acid pickling.
In the BA treatment (reduction annealing), an oxide film, which is referred to as a BA film, is generated at a thickness of approximately 0.001 to 0.1 .mu.m on the surface of the stainless steel product. Antibacterial elements, such as Cu and Ag, in the surface layer of the steel product are not migrated into the BA film by the BA treatment alone, and also, if the surface of the steel product is covered by the BA film, antibacterial elements are not allowed to be dissolved as ions, which may result in a significant decrease in antibacterial properties in the surface portion. Additionally, since the annealing temperature is high in the BA treatment, the antibacterial elements may disappear from the surface layer and a region having a low antibacterial element concentration may be formed in the surface layer, resulting in a decrease in antibacterial properties in the surface portion.
It is an object of the present invention to provide a stainless steel product having superior antibacterial properties and corrosion resistance and a method for producing the same in which the problems associated with the conventional techniques described above are advantageously solved and the antibacterial properties in the surface portion of the end product are enhanced with a relatively low Ag content.